This invention relates to remote access manipulators and more particularly to remote access manipulators for inspecting and repairing nuclear steam generators.
There are many situations in which a hazardous environment limits human access to various locations. One such situation occurs in the inspection and repair of nuclear steam generators. A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and tube sheet, it is important that the U-tubes and tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally it is necessary to either inspect or repair the U-tubes or tube sheet welds by way of access through the primary fluid inlet and outlet plena. For this purpose manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena to perform operations on the U-tubes and tube sheet. However, since the primary fluid which is generally water contains radioactive particles, the inlet and outlet plena become radioactive which thereby limits the time that working personnel may be present therein. Accordingly, it would be advantageous to be able to perform operations of the U-tubes and tube sheet without requiring the presence of working personnel. There are several mechanisms known in the art that attempt to provide a solution to this problem, but none of them have been able to completely solve the problem.
In U.S. Pat. No. 3,913,452 to C. T. Ward et al., issued Oct. 21, 1975 and entitled "Remote Movable Platform" there is described a remotely movable carriage which serves as a mobile platform from which remotely initiated and controlled inspection and work operations might be performed on the tubes in a nuclear steam generator. The carriage includes a stepping mechanism which interacts with a member, such as a tube sheet, relative to which the carriage moves in generally parallel relationship. The stepping mechanism may employ selectively extensible fingers for lateral engagement with the openings in the members. In addition, an extension device may be employed for remotely handling the carriage through the manway during installation and removal. In monitoring the location of the carriage various techniques may be used such as television or, preferably, techniques which initially establish the location of the carriage relative to the tube sheet when first placed against the under surface of the tube sheet and which then plot and monitor the movement of the carriage across the tube sheet surface. While the patent to Ward et al. does describe one type of remote access device, it does not completely solve the problem of remote access operation on members such as tube sheets. For example, should there be a power loss during operation the plotting and monitoring mechanism may not be capable of reestablishing the location of the carriage. Furthermore, should a substantial number of tubes in one area be plugged by deposits, the Ward device might not be able to traverse the plugged area.
Another device for inspecting a tube sheet is described in U.S. Pat. No. 4,004,698 to B. Gebelin issued Jan. 25, 1977 and entitled "Device For Positioning A Member On A Tubular Plate." The device comprises two perpendicular arms capable of relative motion for transporting the member along the tube sheet. While the two perpendicular arms are capable of movement along a rectangular coordinate array of tubes, difficulty would be encountered in avoiding large areas of plugged tubes.